The use of electric current to facilitate the healing of traumatized tissue and broken or fractured bone has been recognized for some time. The stimulative effect of such current appears to occur whether the flow of current is induced naturally, by internal body mechanisms, or artificially, by external sources. While the natural flow of current produced by electrochemical, myoelectric, and piezoelectric-like body mechanisms advantageously facilitates healing without external circuitry, in some instances it is desirable to expedite the recuperative process by artificially supplementing the natural current flow.
A variety of different techniques have been devised for establishing supplemental electric currents in tissue and bone. Briefly, such techniques can be grouped according to both the type of current developed and the manner in which the current is established. Considering first the type of current produced, the current may be characterized as either a direct (DC) current, having an amplitude that is substantially constant as a function of time, or an alternating (AC) current, which exhibits a time-dependent amplitude variation. The use of AC current is preferred because it can be established in a variety of ways, discussed below. DC current, on the other hand, can be induced only by providing a direct electrical connection between the tissue or bone and the energy source.
The manner in which the auxiliary stimulating current is induced offers several alternative forms of classification. First, such techniques can be categorized as being either invasive or noninvasive, depending on the connections provided to the patient. Invasive techniques involve the application of electric current directly to the site of the trauma or fracture through electrodes implanted at the site. While this approach minimizes the electric potential required to generate a particular desired current in the tissue or bone, it also involves the expense and risk of infection attendant surgical implantation procedures. As a result, noninvasive procedures, in which the flow of electric current in the tissue or bone is induced by apparatus external to the patient, are preferred.
The noninvasive establishment of an AC current in tissue or bone can be further grouped according to the electric principle involved in its production. For example, a resistive approach involves the conduction of current directly to the patient through special electrodes coupled to the patient's skin via conductive gel. This technique has the disadvantage of requiring good electrical contact between the electrodes and the skin, necessitating the periodic replacement of the conductive gel on the electrodes.
A second, or inductive, approach employs magnetic fields to establish the desired AC current in the tissue or bone. Specifically, this approach involves the application of an electric current to magnetic coils positioned proximate the patient. The flow of current through the coils produces a magnetic field in the patient's bone or tissue, resulting in the establishment of the desired alternating therapeutic current. This approach has a number of disadvantages including the required use of bulky magnetic coils and an energy source having an output whose frequency and waveform are sufficient to induce the desired stimulating currents at the patient site. The inductive approach also involves relatively large energy losses attributable to the heating of the coil windings produced by the flow of current therethrough.
The third technique for noninvasively establishing an alternating current in the patient's tissue or bone can be referred to as the "capacitive" or electric field approach. This technique typically employs a pair of electrodes that are placed on opposite sides of the treatment site and are insulated from the patient's skin. Energy applied to the electrodes establishes an electric field between the electrodes, normal to the skin. It is this electric field that induces the alternating therapeutic current at the treatment site.
While this approach overcomes the difficulties outlined above with respect to the resistive and inductive techniques, the capacitive production of therapeutically effective levels of electric current at the patient treatment site traditionally presents several additional problems. For example, the patient's skin normally contributes a series capacitive reactance to the equivalent electric circuit representative of the elements between the electrodes. In addition, a much larger variable capacitive reactance is exhibited by the insulative, dielectric "gaps" between the electrodes and the patient.
The combined series capacitive reactance of the elements between the electrodes has been a problem for several reasons. First, this reactive component seriously attenuates the current flow produced by a given potential applied to the electrodes. To understand how this energy loss arises, it may be helpful to briefly review the dynamics of interaction between the stimulating current and the patient. To be therapeutically effective, the electric current must have a frequency that is low enough to ensure its penetration to the site of the fracture or trauma. This requirement is imposed because a high-frequency current will concentrate near the dermal region of the patient in response to a mechanism known as the "electromagnetic skin effect." Other, biological, mechanisms that limit the efficacy of relatively high-frequency current also likely exist.
At therapeutically effective frequencies, the peak energy stored electrostatically in both the electrode-to-skin interface and the epidermis during one cycle of the alternating voltage potential applied to the electrodes is substantially larger than the energy absorbed by the tissue and bone being treated. This stored energy is typically either dissipated within the source or radiated as electromagnetic energy, resulting in a system inefficiency or energy loss. As a result, high levels of reactive power are required to establish the desired therapeutic current.
As an alternative to the use of higher voltages to compensate for energy losses, an inductive reactance can be employed to produce a resonant circuit that reduces the energy losses. For example, a series-connected inductor can be used to recapture the stored energy and apply it to the treatment site during the next cycle of the alternating voltage applied to the electrodes. As a result, only a relatively small amount of energy is dissipated and radiated.
Even with energy losses reduced, the capacitive technique of establishing therapeutic current in tissue and bone still presents several problems. As noted previously, the capacitive reactance exhibited by the dielectric gaps between the electrodes and the patient represents a rather large and variable electrical impedance to the drive circuit. The addition of the inductor to form a resonant circuit having a high quality factor Q, provides a significant reduction in impedance when the circuit is operated at its resonant frequency. Because the capacitive portion of the circuit may vary substantially in response to both the condition of the patient and movement between the electrodes and patient, when a fixed inductance is employed the circuit can be maintained at resonance only by adjusting the frequency of the driver to correspond to the resonant frequency of the circuit as it varies with the changing capacitance. Alternatively, a variable inductive reactance can be used to negate the effect of the changes in capacitance, leaving the resonant frequency of the circuit unchanged.
In U.S. Pat. No. 4,459,988 (Dugot) a circuit is disclosed that employs the former technique. Specifically, a portion of the patient positioned between stimulating electrodes is included in a series resonant circuit incorporating positive feedback to maintain the frequency of the stimulating signal at the resonant frequency of the circuit. The Dugot approach, however, suffers from several disadvantages. First, the disclosed implementation is relatively complex and involves a large number of components. Positive feedback is required to stabilize the circuit with respect to frequency and automatic gain control is preferably employed to regulate the amplitude of the signal generator's output. In addition, the output produced by the circuit may be subject to spurious and multiple high-frequency oscillations decreasing the efficiency of the system. The use of a variable inductive reactance to maintain a constant resonant frequency in the presence of capacitive changes disadvantageously requires the use and expense of some form of adjustable inductor and feedback to control it.
In light of the preceding remarks, it would be desirable to provide a method and apparatus for noninvasively establishing a regenerative electric current in traumatized tissue and broken or fractured bone. It would further be desirable to employ a capacitive technique of establishing such a current that is simple, does not require a high-voltage potential to overcome large variable gap capacitances, is inherently stable with respect to both frequency and amplitude, and rejects high-frequency spurious oscillations.